Enhanced AC immunity in ground fault detection

ABSTRACT

A method for reducing the occurrence of false ground fault detections in a central office terminal is provided. The method includes generating a no-fault signal when no ground current is detected, delaying generation of a fault signal when ground current is detected at least for the duration of an expected pulse in AC induced signal, and when the ground current persists for a sufficient period, generating a signal indicating a fault condition.

BACKGROUND

In many telecommunications applications, repeaters and other electronicdevices are housed in remote units scattered throughout a geographicalregion in the vicinity of a central office. In one example, a remoteunit communicates with the central office and also receives power fromthe central office through the same cable or other communication medium.This cable is also referred to as a “span cable,” “plant,” or “cableplant.” An example of a span cable includes a set of twisted-pairconductors over which telecommunications data is transferred between thecentral office and the remote units, and over which DC power is suppliedby the central office to the remote unit.

The remote unit typically utilizes the power received from the centraloffice over the span cable to power one or more electronic deviceswithin the remote unit. The power delivered via a span cable is oftensusceptible to disturbances (such as faults, voltage spikes and surges)caused by environmental factors such as lighting and nearbyelectrostatic discharges. Left unmitigated, such power disturbances caninterrupt telecommunications operations and permanently damageequipment.

Many electrical protection and personnel safety systems have beendeveloped to detect these disturbances. One such system is genericallyreferred to as ground fault detection system. With ground faultdetection, the system looks for excessive current flowing to ground.When such current is detected, the ground fault detection system takesappropriate action such as shutting down the power supply that transmitspower over the span cable.

AC power lines are often located within the vicinity of the span cableor plant of the telecommunications network. The signals on the AC powerlines can adversely affect signals on the span cable through aphenomenon known as “AC induction.” With AC induction, an AC signal fromthe power lines or other source of AC power is induced onto the copperplant. When the electronic devices of the network are separated by alarge distance, the plant is more susceptible to AC induction.

AC voltages typically are induced longitudinally upon span cables whichcause currents to flow through the longitudinal noise filter circuits toground at both the Central Office Terminal (COT) and Remote Terminal(RT) equipment. The earth ground maintained between the COT and RTinstallation completes the circuit, allowing the induced voltage tomaintain current flow in the communication systems grounding path. Thelongitudinal noise filter circuits present a relatively high impedanceto ground at the AC power line frequencies to avoid large currents fromflowing in the filters ground path, as would be the case in a directcontact of an AC power line with the span cable (known as a power crossevent). The ground fault detection circuit is designed to monitor thelevel of DC current flowing in the grounding system as the result ofleakage currents to ground along the cable span and equipment. ACinduction currents are imposed on the DC leakage currents and can looklike a ground fault to the ground fault detection circuit during thehalf of the AC cycle which is additive to the DC current. Thus, the ACinduced signal could trip the ground fault detection circuit causing thepower supply to be inadvertently turned off. This could be compensatedfor with a large filter, e.g., a large capacitor, in the ground faultdetection circuit to filter out the AC signal. However, the filter wouldhave to be prohibitively large and expensive due to the large voltagesinvolved. Further, if a large capacitor is incorporated into the groundfault detection circuit, any alternating longitudinal voltage on thespan would be exposed to a low (longitudinal) impedance to ground. Ifthe power lines came into direct contact with the cable plant of thetelecommunications network, the power lines would be shorted to groundthrough the network device. Software filters have also been used toattempt to address this phenomenon. However, the effectiveness ofsoftware filters tend to roll off at higher frequencies. It has beendiscovered that some of the most relevant frequencies for AC immunityare harmonics that fall outside the effective range of traditionalsoftware filters.

Therefore, there is a need in the art for enhanced AC immunity in groundfault detection.

SUMMARY

Embodiments of the present invention provide improvements in groundfault detection in a central office terminal. More specifically, in oneembodiment, a method for reducing the occurrence of false ground faultdetections in a central office terminal is provided. The method includesgenerating a no-fault signal when no ground current is detected,delaying generation of a fault signal when ground current is detected atleast for the duration of an expected pulse in AC induced signal, andwhen the ground current persists for a sufficient period, generating asignal indicating a fault condition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a telecommunications system with enhancedAC immunity for ground fault detection according to one embodiment ofthe present invention.

FIG. 2 is a block diagram of one embodiment of an AC immunity circuitaccording to one embodiment of the present invention.

FIGS. 3A and 3B are timing diagrams illustrating one embodiment of aprocess for providing AC immunity to a ground fault detection circuit.

FIGS. 4A and 4B are timing diagrams illustrating one embodiment of aprocess for detecting a ground fault with a ground fault detector withincreased AC immunity.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of illustration specific illustrative embodiments in which theinvention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention, and it is to be understood that other embodiments may beutilized and that logical, mechanical and electrical changes may be madewithout departing from the spirit and scope of the present invention.The following detailed description is, therefore, not to be taken in alimiting sense.

Embodiments of the present invention provide enhanced AC immunity inground fault detection circuits to avoid problems with AC inducedsignals on telecommunication lines. Some embodiments use an AC immunitycircuit that conditions the output of the ground fault detection circuitin a manner that stretches out AC pulses in the ground fault detectionsignal to reduce the chances of a false ground fault detection.

FIG. 1 is a block diagram of one embodiment of a telecommunicationssystem 100 with enhanced AC immunity to ground fault detection. Theembodiment of system 100 is a four wire digital subscriber linecommunication system the includes a central officer terminal 102 and aremote terminal 104 coupled together over a communication medium 105comprising two twisted copper pairs 106 and 108. In other embodiments,the teachings of the present application with respect to AC immunity areapplied to other systems that use copper wires exposed to potentialelectrical disturbances, e.g., single pair systems.

In this embodiment, the central office terminal 102 provides power toand communicates data with the remote terminal 104. The central officeterminal 102 includes communication circuits 110 and 112 thatcommunicate data with corresponding communication circuits 114 and 116,respectively, over twisted copper pairs 106 and 108. In one embodiment,these communication circuits communicate data using high bit ratedigital subscriber line (HDSL), asymmetric digital subscriber line(ADSL), G.SHDSL, or any other appropriate xDSL or other communicationprotocol.

Central office terminal 102 also includes power supply 118 that providespower over communication medium 105 to power remote terminal 104. Powersupply 118 includes two outputs PS+ and PS−. The output of power supply118 is typically a negative voltage on the order of −190VDC. The powersupply 118 injects the power signal on the communication medium throughtransformers 120 and 122 that are coupled to PS+ and PS−, respectively.The power is received in remote terminal 104 at power supply 124. Powersupply 124 is coupled to communication medium 105 through transformers126 and 128. Power supply 124 typically reduces the voltage levelreceived from power supply 118 for use by the circuits of remoteterminal 104, e.g., communication circuits 114 and 116.

Central office terminal 102 also includes circuitry that is designed toprotect the central office terminal 102 from damage due to electricalsurges caused by various natural phenomenon, e.g., lightning strikes.The circuitry 113 appears at each interface of the twisted copper pairs106 and 108 and is connected to ground 131. Protection circuitry 113activate as surge voltages rise above the trigger threshold of theprotection devices used and conduct away large amounts of surgecurrents, thus reducing the surge voltages seen by the end terminalequipment 102 and 104. Further protection and personnel safety circuitryincludes a ground fault detector 130, an AC immunity circuit 132 and aprocessor 134. The ground fault detector 130 is coupled to the powersupply signals PS+ and PS− and is adapted to determine when a current toground 131 exceeds a selected threshold. When such a condition isdetected, the ground fault detector 130 produces a signal that indicatesa ground fault condition has occurred. Unfortunately, the ground faultdetector 130 may provide a false indication of a fault condition due toAC induced signals on the communication medium 105. Thus, the signalfrom ground fault detector 130 is conditioned to reduce the likelihoodof a false indication of a ground fault condition.

This embodiment uses a combination of circuit elements to reduce thepotential for false indications of a ground fault due to AC inducedsignals on communication lines 105. Central office terminal 102includes, for example, a combination of software and hardware filteringalong with circuitry that extends AC pulses in the output of groundfault detector 130. In one embodiment, the hardware filter comprises acapacitor built into the ground fault detector. An example of this typeof hardware filter is shown and described below with respect to FIG. 2.Further, the software filtering is typically implemented in processor134.

AC immunity circuit 132 implements the pulse extender functionality. Forexample, AC immunity circuit 132 receives the ground fault signal fromground fault detector 130. When AC current is present, periodic pulsescorresponding to the various harmonics of the AC fundamental frequencyoccur in the signal from ground fault detector 130. The AC immunitycircuit 132 stretches out the pulses for a period of time sufficient toallow the software filtering of processor 134 to prevent a falseindication of a ground fault condition caused by the higher harmonicpulse rates which exceed the software filters sampling capability. Inone embodiment, the AC immunity circuit 132 stretches out the portion ofthe AC signal that indicates no fault condition such that the output ofground fault detector 130 is conditioned to remain in a no fault statefor a prolonged period.

Processor 134 over-samples the output of AC immunity circuit 132periodically to determine if there is a ground fault in thecommunications system. Without AC immunity circuit 132 or a softwarefiltering means, the processor 134 was prone to false detection ofground faults because it could sample the signal from the ground faultdetector in a low state (active state) caused by the pulses from the ACinduced signal. With some software filtering means, this problem waspartially solved, e.g., at lower harmonic frequencies. However, due tothe presence of higher harmonics in the AC induced signal, e.g.,harmonics above 180 Hz (especially at 540 Hz and 900 Hz), the softwarefiltering could not achieve the sampling rate to eliminate the problem.With the addition of AC immunity circuit 132, the effective samplingrate of the processor is increased beyond the harmonic frequencies ofthe AC induced signal, thereby improving the accuracy of the groundfault detection circuit.

FIG. 2 is a block diagram of one embodiment of a ground fault detector200 and an AC immunity circuit 202 for use in a central office terminal,e.g., central office terminal 102 of FIG. 1. AC immunity circuit 202conditions the output of ground fault detector 200 so as to reduce theeffect of AC induced signals in the telecommunications system.

Ground fault detector 200 detects ground fault conditions. Ground faultdetector 200 includes transistor 204 and resistor 206. Resistor 204 iscoupled to the supply signal PS+. In most embodiments, the PS+ signal isreferenced to ground such that all power signals in the system are belowground potential to prevent corrosion as is known in the art. Resistor206 is also coupled to the emitter of transistor 204. The collector oftransistor 204 is coupled to chassis ground, e.g., earth. The base oftransistor 204 is coupled to the PS− signal through resistor 210. Groundfault conditions cause current to flow in resistor 206 and transistor204. This current is used to indicate a ground fault condition when itrises above a selected level.

Ground fault detector 200 also includes an optocoupler 208 that iscoupled to the base of transistor 204 and the PS+ signal. Optocoupler208 is turned on when current above a selected level flows to earththrough resistor 206 and transistor 204. When this condition isdetected, the output of optocoupler 208 transitions to a low outputvoltage to indicate the fault condition.

When an AC signal is induced on the communication medium, e.g.,communication medium 105 of FIG. 1, this signal causes an AC current toflow in resistor 206 and transistor 204. Thus, on one-half of the ACcycle of the induced signal, optocoupler 208 is turned on and on theother half the cycle optocoupler 208 is turned off. If left unmitigatedor filtered, this AC signal can lead to false indications of a groundfault in ground fault detector 200.

Ground fault detector 200 includes a hardware filter that is used to atleast partially address this problem. The hardware filter, in thisembodiment, comprises capacitor 212 coupled across resistor 206 andtransistor 204. Unfortunately, the value of capacitor 212 cannot be madelarge enough to fully remove the AC components because such a capacitorwould provide a dangerous low impedance path to ground and preventproper electrical protection circuit function. Further, such a capacitorwould be prohibitively large and expensive due to the low frequenciesinvolved with AC signals. Capacitor 212 can be made of sufficient sizeto provide sufficient filtering of only the highest frequency harmoniccomponents (above 1020 Hz) of the AC induced signal. The lower andmiddle frequency components (60 Hz through 900 Hz) are addressed throughpulse extension and software filtering.

Pulse extension is accomplished in AC immunity circuit 202. AC immunitycircuit 202 is coupled to the output of ground fault detector circuit atnode 214. AC immunity circuit 202 conditions this signal at node 214 andprovides an output at node 216.

In one embodiment of AC immunity circuit 202 includes a comparator 218that compares a reference voltage at input 220 with a signal at input222. The reference voltage is established by a voltage dividercomprising resistors 221 and 223 and power supply 225. The signal atinput 222 is the signal from node 214 with pulses caused by induced AC.

AC immunity circuit 202 extends the pulses in the signal at node 214using two signal paths with different time constants. The two signalpaths control the charging and discharging of capacitor 224. The firstsignal path charges capacitor 224. The first signal path includesresistor 226 and diode 228. Resistor 226 is coupled between node 214 anda power supply 225, e.g., a 3.3 V supply. Diode 228 is coupled betweennode 214 and node 222. Capacitor 224 is coupled between node 222 andground.

The second signal path controls the discharging of capacitor 224. Thesecond signal path includes resistor 230 coupled between nodes 214 and222. Resistor 230 has a resistance value that is substantially greaterthan resistor 226. This difference in resistance values controls thedifference in time constants between the two paths. In one embodiment,resistor 226 is 10 KΩ and resistor 230 is 75 KΩ.

The operation of the circuit of FIG. 2 is described with respect to thetiming diagrams of FIGS. 3A, 3B, 4A, and 4B. FIGS. 3A and 3B illustratethe conditioning effect of AC immunity circuit 202 on the output ofground fault detector 200 in the presence of an AC induced signal.Further, FIGS. 4A and 4B illustrate the manner in which AC immunitycircuit 202 processes a signal without an AC induced component.

FIG. 3A illustrates a signal at node 214 when an AC signal is induced onthe communication medium, e.g., communication medium 105 of FIG. 1, by aco-located power line. At time T1, the ground fault detector signal 214is at a high voltage level corresponding to the time where the inducedAC current opposes the DC leakage current, resulting in a net currentbelow the selected level of the ground fault detector. At time T2, thesignal output by the ground fault detection circuit transitions to a lowvoltage which corresponds to the point where the induced AC currentbecomes additive to the DC leakage currents and exceeds the selectedlevel of ground fault detector 200. In the absence of AC induction, acontinuous low voltage state of this signal would indicate that a DCground fault has been detected.

During this cycle (between T2 and T3), the voltage at node 214 isgrounded and the capacitor 224 is enabled to discharge through resistor230, e.g., through the second signal path. Because the time constant ofthe second signal path is much longer than the time constant of thefirst signal path, the voltage at node 222 does not change significantlybefore it is recharged as described below.

The first signal path of AC immunity circuit 202 maintains the no faultstate during the other half of the AC cycle of the signal at node 214.At time T3, the signal shown in FIG. 3A returns to a high voltage level.This turns on the diode 228 and allows the capacitor 224 to be chargedfrom voltage source 225 through resistor 226 and diode 228. Because theresistor 226 is selected with a lower resistance value, the capacitor isquickly charged up to a level sufficient to maintain the indication ofno fault condition.

The AC immunity circuit 202 produces the conditioned ground faultdetection signal based on the voltage across capacitor 224 appearing, atnode 222 which is the input to comparator 218. A sample of comparator218 output at node 216 of AC immunity circuit 202 is shown in FIG. 3B.As can be seen in FIG. 3B, the output voltage at node 216 remainsconstant at a level indicating no fault despite the AC induced signal onthe communication lines. The voltage at node 222 is maintained above thereference voltage set at node 220 by the resistor divider. Thus,comparator 218 produces the high voltage output at node 216. Thisindicates to the processor that there is no fault despite the AC inducedsignal.

When the DC leakage current increases above the selected level, a trueDC ground fault conditions occurs. Increased DC leakage currentdecreases the high times and increases the low times in signal 3A,allowing the second signal path of AC immunity circuit 202 to dischargecapacitor 224. The resulting voltage on node 222 falls below theselected level on node 220 forcing comparator 218 output 216 to go low,AC immunity circuit 202 produces a slightly delayed signal indicatingthe ground fault condition as shown in the timing diagrams of FIGS. 4Aand 4B. This delay is caused by the time constant of the second path inAC immunity circuit 202 that compensates for the AC induced signal. Inthis example, a ground fault occurs at time T5. At time T5, the signalfrom the ground fault detector transitions from a high (no fault)condition to a low (fault) condition. At this time, diode 228 is turnedoff and capacitor 224 is slowly discharged through resistor 230. Whenthe capacitor voltage drops below the reference voltage at node 220, thecomparator 218 trips and changes the output at node 216 to a low voltagelevel indicating a ground fault at T6.

It is noted that the described embodiments have used a low voltage levelto indicate a fault condition. It is understood that in otherembodiments, a fault condition is indicated by a high voltage signal.Further, capacitors 242 and 244 are standard bypass noise capacitors. Inone embodiment, resistor 250 is included as a pull-up resistor becausecomparator 218 is open collector. Further, optional resistor 248 can beincluded to add hysteresis to comparator 218, but, it is not necessaryto improve noise performance. Further, with the use of AC immunitycircuit 202, the processor, in some embodiments, does not implement asoftware filter on the output of AC immunity circuit 202.

In another embodiment, AC immunity circuit 202 extends any AC pulse thatis present in the output of the ground fault detector 200 through theuse of a retriggerable monostable timer to decrease the chances of afalse positive indication of a ground fault. The pulse is extended atleast for the duration of the low voltage cycle of the AC inducedsignal. In one embodiment, the pulse is stretched past the edge of thenext sampling period. This effectively raises the bandwidth of thesoftware filter.

1. A central office terminal, comprising: at least one communicationcircuit adapted to communicate signals with at least one remote unitover at least one communication medium; a power supply adapted toprovide power to the remote terminal over the at least one communicationmedium; a ground fault detection circuit, coupled to the power supply todetect ground fault current; an AC immunity circuit, coupled to theground fault detection circuit, that is adapted to modify the output ofthe ground fault detection circuit to reduce the occurrence of falseground fault detections due to AC induction; and a processor, coupled tothe power supply and the AC immunity circuit, that is adapted todetermine when a ground fault occurs based on the signal from the ACimmunity circuit.
 2. The central office terminal of claim 1, wherein theground fault detection circuit comprises: a resistor coupled to a powersupply; a bipolar junction transistor coupled between the resistor andchassis ground; and an optocoupler, coupled to the base of the bipolarjunction transistor and to the resistor.
 3. The central office terminalof claim 1, wherein the AC immunity circuit comprises: a voltagereference; a comparator with one input coupled to the voltage reference;a pulse extender circuit, coupled to another input of the comparator,the pulse extender circuit coupled to receive a signal from the groundfault detection circuit, and to extend the duration of pulses in ACinduced signals in the signal from the ground fault detection circuit;and wherein the comparator is adapted to produce an enhanced groundfault detection signal based on a comparison of the voltage referenceand the output of the pulse extender circuit.
 4. The central officerterminal of claim 1, wherein the processor is adapted to implement asoftware filter to filter a signal from the AC immunity circuit.
 5. Thecentral office terminal of claim 1, wherein the at least onecommunication medium comprises two twisted copper pairs.